The present invention relates to an electrostatic coater (or painting device) and, in particular, to adjustment of frequency of a high-frequency low voltage supplied to an electrostatic painting device with a high-voltage booster circuit.
As is also disclosed in Japanese Patent Application Public-disclosure No. 10-128170, an internal booster-type electrostatic spray gun incorporating a high-voltage booster circuit has been developed as an electrostatic painting device. Such an electrostatic painting device, as is schematically described in FIG. 1, consists of a high-frequency low-voltage generator 1, an electrostatic spray gun (electrostatic painting device body) 2, a low-voltage cable 3, an air supplier (which is not shown) and a paint material supplier (which is not shown). A high-voltage booster circuit 201 comprises a transformer 202, a multiple voltage rectifier circuit 203, a resistor 204 and an output terminal 205. The high-frequency low-voltage generator 1 converts a voltage from a commercial alternating-current power supply to a DC voltage of 12V via a rectifier 101 and DC-DC converter 102. The thus obtained DC voltage is supplied to the intermediate point of the primary side coil of the transformer 202 via a line 103 and low-voltage cable 3. The ends of the primary side coil are connected to the collectors of transistors 104 and 105, respectively, via the low-voltage cable 3, and their emitters are grounded by a common line 106. From an oscillation control circuit 107 to the bases of the transistors 104 and 105 are provided driving signals which are in 180-degree phase shift with each other, whereby the transistors 104 and 105 are turned on alternately at frequencies of the driving signals. The multiple voltage rectifier circuit 203, resistor 204 and output terminal 205 are connected to the secondary side coil of the transformer 202. The transformer 202 boosts the primary side voltage by a dozen times, which is further boosted by the multiple voltage rectifier circuit 203 (by ten times in this example) to obtain a DC voltage of xe2x88x9240 kv xcx9cxe2x88x9290 kv.
The high-voltage booster circuit incorporated in the internal booster-type spray gun has an intrinsic parallel resonance frequency (frequency at which a consumed current becomes minimum: hereafter referred to as an anti-resonant frequency) attributable to its unique hardware structure, and when a voltage of such an anti-resonant frequency is supplied to a high-voltage booster circuit, power can be converted to high voltages most efficiently. In other words, when a voltage of an anti-resonant frequency is supplied, a current consumed at a high-voltage booster circuit is small, whereby the life of a transformer can be maximized while a load to be caused on the spray gun can be minimized. Further, as it is possible to generate a maximum voltage, efficient utilization of a voltage becomes viable.
FIG. 2 is a graph representing a change in current I consumed by a high-voltage booster circuit of an electrostatic spray gun when frequency f of an alternating-current low voltage sent from a high-frequency low-voltage generator to the high-voltage booster circuit is varied, and representing a change in boosted negative DC voltage V. As can be seen from FIG. 2, the DC voltage V does not change much in the neighborhood of the anti-resonant frequency, whereas the current I changes significantly. In this example, when the device is driven at frequencies at which the consumed current I exceeds approximately 1 A, the transformer is likely to be damaged by heat. Therefore, it is ideal that the device be driven at a driving frequency f0 at which the consumed current I becomes a minimum, that is, about 0.2 A.
Dispersion arising during the manufacture of high voltage booster circuits (for example, dispersion in electronic components of circuits) sometimes results in disadvantageous fluctuation of an intrinsic anti-resonant frequency of a high-voltage booster circuit. Further, when voltage supply from a high-frequency low voltage generator shifts from a high-voltage booster circuit for generating a voltage of, for example, xe2x88x9240 kv to a high-voltage booster circuit for generating a voltage of, for example, xe2x88x9290 kv, an optimum transmission frequency cannot be specified. Still further, when a technical specification of a high-voltage booster circuit per se is changed, for example, a transformer thereof is improved or modified with a view to cost reduction, etc., an anti-resonant frequency specific to the high-voltage booster circuit also changes.
If a high frequency low voltage whose frequency does not coincide with an anti-resonant frequency specific to a high voltage booster circuit is supplied to the high voltage booster circuit, an excess current flows into a transformer of the high voltage booster circuit to cause failure, and a rated output is not generated. Therefore, when an intrinsic anti-resonant frequency fluctuates beyond the referential scope as a result of dispersion arising during manufacture of a high-voltage booster circuit, an electrostatic spray gun incorporating the high voltage booster circuit is considered to be defective and thus, productivity substantially declines.
On the other hand, a volume for adjusting a frequency may be attached to the oscillation control circuit 107 of the high frequency low voltage generator 1 indicated in FIG. 1 to initialize an oscillation frequency at the time of assembly of the high frequency low voltage generator 1. For example, a transmission frequency is set to be about fx, in the case of a high voltage booster circuit cartridge for xe2x88x9260 kv (natural anti-resonant frequency=fx), whereas a transmission frequency is set to be about fy in the case of a high voltage booster circuit cartridge for xe2x88x9240 kv (natural anti-resonant frequency=fy). When anti-resonant frequencies specific to high voltage booster circuits disperse, an ammeter is connected to the line 103 of the high frequency low voltage generator 1. A volume is adjusted by monitoring a current value read by the ammeter to set, as an intrinsic anti-resonant frequency, a frequency at which the current value becomes minimum. However, initialization or resetting of a frequency while monitoring an ammeter can be troublesome.
Given the aforementioned problems of the prior art, it is an object of the present invention to provide an electrostatic painting device with a transmission frequency adjustment device which can automatically adjust a transmission frequency such that a consumed current running in the high voltage booster circuit does not exceed a certain value.
An electrostatic painting device provided with a transmission frequency adjustment device of the present invention comprises a high voltage booster circuit provided inside the body of the electrostatic painting device to rectify a high frequency low voltage and generate a DC high voltage for electrostatic painting, a high frequency low voltage generator provided independent of the body of the electrostatic painting device to generate a high frequency low voltage, a low voltage cable connecting the high frequency low voltage generator to the high voltage booster circuit, current sensor for detecting a current value corresponding to a value of an intrinsic consumed current at the high voltage booster circuit, and a frequency control device for adjusting a frequency of a high frequency low voltage such that the value of the current detected by the current sensor does not exceed a certain value.
According to an embodiment of the present invention, the frequency control device exercises control for determining a driving frequency of the high voltage booster circuit such that a value of the current detected by the current sensor becomes a minimum value. The current sensor is installed in the high frequency low voltage generator to detect a current guided to the low voltage cable. The frequency control device can operate either when a power switch of the electrostatic painting device is closed or at the set times. The electrostatic painting device is further provided with an abnormality indication device for indicating an abnormality when the value of the current detected by the current sensor exceeds a predetermined value. The frequency control device adjusts a frequency of a high frequency low voltage when an abnormality is indicated.